Cardiovascular disease (CVD) was blamed for 37% of the 2.4M deaths in the US (2003) [1]. CVD is the leading cause of death in the US and the developed world.
Currently Available Drug-Eluting Stents (DES) Pose a Major Potential Health Concern
The clinical use of Drug Eluting Stents (DES), in relation to Bare Metal Stents (BMS), has evolved over a period of approximately 18 months from approximately 0% usage in the U.S., to the point where they were used in approximately 80% of coronary stent procedures in the U.S. [2, 3].
The above cited recent studies indicates that there is a significant, growing population (approximately 6 Million individuals worldwide [4]) who currently find themselves having been implanted with DES and face a choice between taking the expensive and risky drug clopidogrel—potentially for life—or increased risk of premature death.
The Vascular Smooth Muscle Cell, VCAM-1 and Rapamycin: Vascular SMC Proliferation Contributes to Angioplasty-Induced Stenosis and in-Stent Restenosis.
The primary function of the vascular SMC in adult animals is contraction and SMCs express a unique repertoire of genes that allow for this specialized form of contraction, including SM α-actin, smooth muscle myosin heavy chain (SMMHC), SM22α, calponin, desmin, smoothelin—genes we refer to as SMC differentiation marker genes [5-8]. This repertoire of genes is typically used to describe the “contractile” phenotype or mature SMC.
VCAM-1 is a Marker of the Phenotypically Modified/Proliferating SMC.
The changes in SMC gene expression profiles associated with injury-induced phenotypic modulation are transient. That is, SMCs undergo phenotypic modulation as a natural response to repair the injured blood vessel, transitioning from a contractile phenotype to a synthetic phenotype but revert back to a contractile phenotype as the lesion resolves itself. Thus, this continuum of altered SMC gene expression profiles can be used to target the phenotypically modified SMC that invests in the developing neointima using molecular targeting. VCAM-1 (vascular cell adhesion molecule 1) is expressed in proliferating SMCs [9, 10] and transiently upregulated in SMCs following acute vascular injury and in atherosclerotic lesions [11]. The function of VCAM-1 is to promote cell-cell interaction required for SMC migration and recruitment or attraction of other cell types into the lesion, e.g. VCAM-1 interaction on SMCs with integrins on leukocytes, monocytes or macrophages (all inflammatory cells) [9]. Because VCAM-1 is expressed at much lower levels in the quiescent contractile SMC phenotype, but increased in proliferating SMCs, VCAM-1 can thus be used to target the proliferating SMC.
Rapamycin is a Potent SMC Anti-Proliferative Agent and the Bench-Mark Agent for Preventing in-Stent Restenosis by Release from a DES.
The cell cycle consists of 5 basic steps: dormancy (G0) or the contractile SMC phenotype, gap phase 1 (G1), synthesis (S), pre-mitosis or gap phase 2 (G2) and mitosis (M). In response to acute vascular injury, SMCs leave G0 and enter G1 to begin the process of cell proliferation and division into M phase; this is the synthetic migratory or proliferative SMC phenotype. The strategies for preventing SMC proliferation and entry into the cell cycle have been to block various phases of the cell cycle once the cell has left G0 in response to injury or some acute growth stimulus. Sirolimus, or rapamycin, and its analogues, ABT578 (Abbot Pharmaceuticals) and everolimus, are immunosuppressants with both anti-inflammatory and antiproliferative properties that interfere early in the cell cycle by inhibiting the passage of cells from G1 to S phase. Drugs that inhibit cell cycle in the G1 phase are considered cytostatic and may be less toxic than drugs that act later in the cell cycle [12, 13]. Rapamycin is the most thoroughly investigated agent of this group and has become the bench-mark agent for the prevention of coronary artery restenosis [14]. Thus, because rapamycin is considered “cytostatic”, SMCs treated with rapamycin do not die but maintain their viability in the growth arrested state.
Molecular Targeting of Microbubble Carriers
Recent research has investigated the feasibility of targeted ultrasound contrast microbubbles as a means of detecting intravascular manifestations of disease. Pathology is often accompanied by alterations of the endothelial cell layer lining of the affected blood vessels. This dysfunction may occur in the microcirculation, and is identified by the selective expression or up-regulation of certain molecules on the vascular endothelial surface. Many of the molecular markers of endothelial dysfunction corresponding to disease states such as atherosclerosis [15], transplant rejection [16], inflammation and ischemia reperfusion injury [17] are well characterized. However, there is currently no non-invasive, clinically approved technique to assess the extent and location of such vascular pathologies. Experimental formulations of targeted microbubbles, which contain a surface-bound ligand specific for the intended target, are injected intravascularly and, after a short circulation period, are observed to accumulate at the target site. Subsequent ultrasound imaging enables determination of the location and extent of the targeted disease state [18]. This technique, known as “targeted contrast enhanced ultrasound”, may achieve high spatial resolution, real time imaging, and a linear or other measurable correlation between adherent microbubbles and the received signal.
There is therefore a need for, among other things, the drug, the drug carrier, and the means of localizing delivery; and a means to guide the focal delivery under real time image guidance.